Chapter 19b

The Kidneys

Reabsorption

• Principles governing the tubular reabsorption of solutes and water

Figure 19-11

Na+ is reabsorbed by active transport.

Electrochemical gradient drives anionreabsorption.

Water moves by osmosis, followingsolute reabsorption.

Concentrations of other solutesincrease as fluid volume in lumendecreases. Permeable solutes arereabsorbed by diffusion.

Na+

Anions

H2O

K+, Ca2+,urea

Tubularepithelium Extracellular fluidTubule lumen

Filtrate is similar tointerstitial fluid.

1

2

3

4

1

2

3

4

Reabsorption

• Transepithelial transport• Substances cross both apical (lumen side) and

basolateral membrane

• Paracellular pathway• Substances pass through the junction between

two adjacent cells

Reabsorption

Figure 19-12

[Na+] high [Na+] high[Na+] low

K+Na+ ATP

Proximal tubule cellInterstitial

fluidTubulelumen

Na+ reabsorbed Na+ enters cell through membrane proteins,moving down its electrochemical gradient.

Na+ is pumped out the basolateral sideof cell by the Na+-K+-ATPase.

ATP = Active transporter= Membrane protein

KEY

Na+

Filtrate is similar tointerstitial fluid.

12

1

2

• Sodium reabsorption in the proximal tubule

Reabsorption

• Sodium-linked glucose reabsorption in the proximal tubule

Figure 19-13

= SGLT secondary active transporter

= GLUT facilitated diffusion carrier

[Na+] high[glu] low

Na+ Na+

[Na+] high

[glu] low

glu glu

[Na+] low[glu] high

K+ATP

Glucose and Na+

reabsorbed+

Na+ moving down its electrochemical gradientusing the SGLT protein pulls glucose into thecell against its concentration gradient.

Glucose diffuses out the basolateral side ofthe cell using the GLUT protein.

Na+ is pumped out by Na+-K+-ATPase.

Proximal tubule cell Interstitial fluidTubule lumen

ATP = Active transporter

KEY

Filtrate is similar tointerstitial fluid.

1

2

3

1

2

3

Reabsorption

• Urea • Passive reabsorption

• Plasma proteins • Transcytosis

Reabsorption

• Saturation of mediated transport

Figure 19-14

Renal threshold isplasma concentrationat which saturationoccurs.

Transport maximum (Tm) is transportrate at saturation.

Saturation occurs.

Plasma [substrate] (mg/mL)

Tran

spo

rt r

ate

of

sub

stra

te (

mg

/min

)

Reabsorption

• Glucose handling by the nephron

Figure 19-15a

Reabsorption

Figure 19-15b

Reabsorption

Figure 19-15c

Reabsorption

Figure 19-15d

Secretion

• Transfer of molecules from extracellular fluid into lumen of the nephron • Active process

• Important in homeostatic regulation• K+ and H+

• Increasing secretion enhances nephron excretion

• A competitive process• Penicillin and probenecid

Excretion

• Excretion = filtration – reabsorption + secretion

• Clearance • Rate at which a solute disappears from the

body by excretion or by metabolism• Non-invasive way to measure GFR• Inulin and creatinine used to measure GFR

Inulin Clearance

• Inulin clearance is equal to GFR

Figure 19-16

Glomerulus

Peritubularcapillaries

Afferentarteriole

Efferentarteriole

Nephron

Filtration(100 mL/min)

= 100 mL ofplasma or filtrate

Inulin concentrationis 4/100 mL.

GFR = 100 mL /min

100 mL plasma isreabsorbed. No inulinis reabsorbed.

100% of inulin isexcreted so inulinclearance = 100 mL/min.

Inulinmolecules

100 mL,0% inulin

reabsorbed

Inulin clearance = 100 mL/min100% inulin

excreted

KEY

1

2

3

4

1

2

3

4

Inulin Clearance

Figure 19-16, steps 1–4

Glomerulus

Peritubularcapillaries

Afferentarteriole

Efferentarteriole

Nephron

Filtration(100 mL/min)

= 100 mL ofplasma or filtrate

Inulin concentrationis 4/100 mL.

GFR = 100 mL /min

100 mL plasma isreabsorbed. No inulinis reabsorbed.

100% of inulin isexcreted so inulinclearance = 100 mL/min.

Inulinmolecules

100 mL,0% inulin

reabsorbed

Inulin clearance = 100 mL/min100% inulin

excreted

KEY

1

2

3

4

1

2

3

4

GFR

• Filtered load of X = [X]plasma GFR

• Filtered load of inulin = excretion rate of inulin

• GFR = excretion rate of inulin/[inulin]plasma = inulin clearance

• GFR = inulin clearance

Excretion

Table 19-2

Filtration(100 mL/min)

Glucosemolecules

100 mL,100% glucose

reabsorbed

Glucoseclearance

= 0 mL/min

(a) Glucose clearance

No glucoseexcreted

Plasma concentrationis 4/100 mL.

GFR = 100 mL /min

100 mL plasma isreabsorbed.

Clearance depends onrenal handling of solute.

KEY

= 100 mL ofplasma or filtrate

1

2

3

4

1

2

3

4

Excretion

• The relationship between clearance and excretion

Figure 19-17a

Excretion

Figure 19-17b

(b) Urea clearance

Plasma concentrationis 4/100 mL.

GFR = 100 mL /min

100 mL plasma isreabsorbed.

Clearance depends onrenal handling of solute.

KEY

Filtration(100 mL/min)

Ureamolecules

100 mL,50% of ureareabsorbed

50% of ureaexcreted

Ureaclearance

= 50 mL/min

= 100 mL ofplasma or filtrate

1

2

3

4

1

2

3

4

Excretion

Figure 19-17c

(c) Penicillin clearance

Plasma concentrationis 4/100 mL.

GFR = 100 mL /min

100 mL plasma isreabsorbed.

Clearance depends onrenal handling of solute.

KEY

Filtration(100 mL/min)

100 mL,0 penicillinreabsorbed

Someadditionalpenicillinsecreted.

Penicillinclearance =150 mL/min

More penicillinis excreted than

was filtered.

Penicillinmolecules

= 100 mL ofplasma or filtrate

1

2

3

4

1

2

3

4

Gout

• Limit animal protein. Avoid or severely limit high-

purine foods, including organ meats, such as liver, and herring, anchovies and mackerel. Red meat (beef, pork and lamb), fatty fish and seafood (tuna, shrimp, lobster and scallops) are associated with increased risk of gout. Because all animal protein contains purines, limit your intake.

• Eat more plant-based proteins. You can increase your protein by including more plant-based sources, such as beans and legumes. This switch will also help you cut down on saturated fats, which may indirectly contribute to obesity and gout.

• Limit or avoid alcohol. Alcohol interferes with the elimination of uric acid from your body. Drinking beer, in particular, has been linked to gout attacks

Micturition

• The storage of urine and the micturition reflex

Figure 19-18a

Bladder(smooth muscle)

Internal sphincter (smoothmuscle) passively contracted

External sphincter (skeletal muscle) stays contracted

(a) Bladder at rest

Tonicdischarge

Relaxed(filling)state

HigherCNSinput

Incontinence

Micturition

Figure 19-18b

Stretch receptors fire.

Parasympathetic neurons fire.Motor neurons stop firing.

Smooth muscle contracts.Internal sphincter passivelypulled open. External sphincterrelaxes.

(b) Micturition

Internal sphincter

External sphincter

Tonicdischargeinhibited

Higher CNSinput may

facilitate orinhibit reflex

Sensory neuron

Parasympatheticneuron

Motor neuron

Stretchreceptors

1

2

3

1

2

3

23

Summary

• Functions of the kidneys• Anatomy• Kidney, nephron, cortex, and medulla• Renal blood flow and fluid flow from glomerulus

to renal pelvis

• Overview of kidney function• Filtration• Podocytes, filtration slits, and mesangial cells• Filtration fraction, GFR, and regulation of GFR

Summary

• Reabsorption• How solutes are transported• Transport maximum and renal threshold

• Secretion• Excretion• Clearance, inulin, and creatinine

• Micturition